One example of a conventional brake pressure control apparatus is disclosed in U.S. Pat. No. 4,744,610, issued on May 17, 1988. The conventional apparatus disclosed in that patent includes a brake circuit for connecting a master cylinder to a front wheel cylinder, a normally open valve unit installed in series in the brake circuit, a return circuit connected in parallel to the brake circuit and a normally closed valve unit installed on the return circuit. Additionally, a reservoir unit and a pump unit are installed in the return circuit and a controlling unit is provided for controlling the operations of the normally open valve unit, the normally closed valve unit and the pump unit. A regulating valve unit is connected to the front wheel cylinder for regulating the pressure applied to the rear wheel cylinder in response to the pressure applied to the front wheel cylinder.
In the aforementioned conventional apparatus, the pump unit must return the brake fluid from the front and rear wheel cylinders to the master cylinder. Therefore, the pump unit must have a large capacity in order to return the required quantity of brake fluid. Unfortunately, the large pump unit is difficult to install in the vehicle due to space limitations. Moreover, from a cost standpoint, the large pump can be disadvantageous. Further, when the brake pressure is increased in the front wheel cylinder, the brake pressure is also increased in the rear wheel cylinder. Therefore, the brake pressure increases gradually in the front and the rear wheel cylinders, and thus, brake pressure response is deteriorated.